1. Field of the Invention
The present invention relates to a pressure sensing apparatus and, more particularly, to a pressure sensing apparatus for measuring ice pressure forces which is relatively insensitive to the stiffness or elasticity of the ice formation.
2. Description of the Prior Art
Loads are traditionally measured by point application load cells. A point load is transposed onto a rigid member of a cell which in turn measures the total applied force. Unfortunately, pressure (force per unit area) is more difficult to measure particularly on a large scale, i.e. geotechnical pressures, since it requires a fairly large sampling area to accurately detect an average pressure. Frequently, such pressures are measured by resolving or summing the pressure over a defined area into a single point load and, thereafter, employing a conventional load cell. However, the accuracy of this method depends entirely on the particular method used to resolve the pressure forces. Traditionally, the pressure is resolved into a concentrated load by permitting the pressure to act against a large plate. The total force against the plate is then measured using a rigid arm from a load cell. Unfortunately, the structural characteristics or properties of the plate play a vital role in determining the accuracy of the results. In other words, the bending properties of the plate may unduly interfere with the accuracy of the results since the pressure force will not be uniformly distributed over the entire plate.
The measurement of pressure forces within an embedding medium is particularly important in determining geotechnical pressures as well as developing design criteria. More particularly, the measurement of ice pressure forces is important in determining environmental design criteria for Arctic offshore and coastal structures. Generally, the failure strength of ice, in situ, the stress history of a typical ice park, and the modes of interaction between the structure and the ice pack are the minimum amount of data required to begin the design of an arctic offshore or coastal structure. It is particularly important to gather the essential design data in situ. Samples removed from the embedding medium for subsequent laboratory testing are of limited value since the environmental restraints once removed are difficult if not impossible to accurately recreate in a laboratory. This is particularly true for ice samples which are extremely vulnerable to outside interference once removed. In addition, measuring pressures in situ is particularly valuable since it reflects what the embedding medium is actually experiencing in terms of pressure forces. Therefore, the need exists for a reliable pressure sensing apparatus which can gather the design data from the ice formations in place.
Industry has recognized several factors peculiar to ice pressure measurement which render the prior art inoperable. Due to the crystalline structure of ice, the sensing area of the apparatus must be large with respect to the grain dimensions in order to accurately measure a pressure force. Grain sizes have been measured several inches in size. Therefore, a fairly large area is required to generate an average pressure. In addition, the effective stiffness of the pressure sensor should not cause the localized brittle fracture or plastic deformation of the ice adjacent the sensing plates. In other words, the effective stiffness of the sensor should be as close to the average anticipated stiffness or elasticity of the ice as possible. Due to large temperature fluctuations in the polar regions, differential thermal movement between the sensor and the ice occurs. This creates an artificial pressure which can and should be minimized by design.
The modulus of elasticity of ice is not constant. It varies from 40,000 to 1,000,000 psi depending on the temperature, salinity, grain structure, etc. Therefore, while the effective stiffness of the sensor should be close to the average anticipated stiffness of the ice to prevent localized stiffness problems, the sensor should be relatively insensitive to sudden variations in ice stiffness. The sensor must also be insensitive to the occurrence of ice creep which can affect the accuracy of an embedded ice sensor. In addition, the sensor must include measuring components such as electrical wire resistance strain gauges which are located within a circuit to compensate for direct thermal effects. The sensor must also be water proof to prohibit the formation of ice within the sensor which otherwise would interfere with the functioning of the sensor's internal components. The sensor must also be able to withstand the high pressure forces, i.e. 200-500 psi, exerted by the ice formation.
The use of a thin sensor with respect to its width is preferred. "On Recording Stresses In Ice" by Metge, M., Strilchuk, A., and Trofimenkoff, P., Proceedings of the Third International Symposium on Ice Problems, Aug. 18-21, 1975, Hanover, N.H., discloses the use of a wide, thin and soft sensor to satisfy the above-recognized problems associated with ice pressure measurements. Metge recommends the use of a sensor whose stiffness is substantially less than the stiffness of ice. Metge discloses a sensor having an aluminum plate sandwiched between two layers of an elastomeric material which is in turn sandwiched between two outer aluminum plates. The amount of deformation in the sensor is determined by measuring the change in capacitance between the inner and outer aluminum plates. A correlation is made between the change in capacitance and the applied pressure. Metge fails to disclose a temperature compensating feature with the use of capacitors. Furthermore, Metge fails to disclose the use of a flexible member displaced between two aluminum plates by a series of standoffs or ribs having a plurality of strain gauges attached to the member between the standoffs for measuring strain in the member resulting from the reverse curvature bending of the member.